A typical electrical service for a modern household includes a minimum 150-amp. 220 V. installation comprising a service head hookup from utility company power lines, a usage meter, an entrance service panel (also referred to a "fuse box") including protective devices such as fuses and/or circuit breakers, and multiple branch circuits for distributing electrical power throughout the house to appliances, lighting and wall outlets. The circuits characteristically employ one or more "hot" wires, a "neutral" wire and usually a "ground" wire combined in a single sheathing. All wires, except for the ground wire, are normally individually insulated to keep them from touching and causing a short.
One of the most common wires used in household electrical systems is type T wire which is wrapped in thermal plastic insulation. The usual cabling is NM or nonmetallic sheath cable (commonly called "Romex") comprising two or more individually insulated type T wires and a bare copper grounding wire, all coated with a plastic sheath. The wires may be directly embedded in the cable itself (type NMC or UF cable) or have space filled with jute (type NM cable). Household circuits may also employ armored cable (commonly called "BX") wherein the wires are wrapped in heavy paper and surrounded by a spiral sheath of flexible steel or aluminum. Armored cable is rarely found in newly constructed homes.
Protection for current overloads and short circuits is conventionally provided by fuses and circuit breakers. Fuses generally comprise elongated metal wires through which the electrical current is caused to flow and which melt ("blow") when too much current is applied. This results in a gap or "open circuit" through which electricity can no longer flow, thus cutting off power. The fuse is designed to blow if the electricity requirements of the appliances and other electrical devices plugged into a particular circuit exceed the amperage rating of that circuit. The fuses will also blow if wires directly touch one another causing a short circuit. Circuit breakers are switches that act similarly to fuses for protecting circuits. However, instead of permanently severing a wire when an overload occurs, the breaker is "tripped" to switch from an "on" to an "off" position. Once the overload problem has been corrected, the breaker can simply be reset by switching it back to the "on" position.
While fuses and circuit breakers both provide protection for current overloads and short circuits, neither provides protection against the "partial short circuit." This condition exists when a current load that does not reach the overload rating of the circuit's conventional protection flows through the wires, but nevertheless causes excessive heat sufficient to present a risk of fire. Three examples of occurrences of this problem are as follows:
1. A mouse chewed off properly rated, nonmetallic-sheathed "Romex" cable in an attic of a house. A hot electrical wire partially shorted to the ground causing a fire. Fortunately, the wire melted apart and terminated the current flow, before any serious damage occurred. Electricity to the load side was interrupted, but the circuit breaker did not trip.
2. An air conditioner compressor fan switch mounted on the side of a house corroded causing the outside unit to not function. Upon throwing the switch to check for proper operation, the switch's internal components shorted causing a fire on the side of the house. The fire continued until the 60-amp. circuit breaker in the garage was tripped. Upon closer examination, it was observed that the wires going to the outside switch melted off the switch and continued to stay shorted as insulation melted away.
3. A telephone installer drilled a hole in the side of a house to install a jack. The main electrical service for the house was on the other side of the wall. The drill bit shorted the main feed to the home service panel box inside the house. The box became so hot that it set fire to the surrounding paneling. Aerial wire to the house became so hot that it broke away from the house and continued to stay shorted setting fire to the yard.
A ground fault circuit interrupter (GFCI) provides protection against shock due to a "ground fault." A ground fault can occur when a bare hot wire touches a grounded wire, an armored cable, a metal conduit or the current is otherwise redirected from its normal path to ground. The GFCI monitors the equal current in the "hot" and "neutral" wires, and shuts off all current if there is a drop in the return current. Receptacle versions of GFCI devices, if installed on a first receptacle of a circuit, will usually also protect against ground faults that might occur at other receptacles or devices in the same circuit. GFCIs are also available that are installed directly in the breaker panel and combine a breaker switch with a GFCI. Such combination protects against overloads, shorts and ground faults. The partial short circuit discussed above, which overheats a wire and may cause a fire, may not result in an inequality between outgoing and return currents, so may not trip the GFCI.